1. Field of the Invention
This invention relate to a composite optical semiconductor device including a light emitting element and a light detecting element formed in one body.
2. Description of the Prior Art
A light emitting diode (LED) has an output characteristic exhibiting temperature dependent fluctuations smaller than that of a semiconductor laser device. The LED output characteristic is also more linear. Further, a LED is a source or light easy to use, except for the problems of power level and response speed. But, if the LED is actually used in an analog transmission system, nevertheless the fluctuation of its output characteristic must be compensated.
Nowadays, a wideband opto-electric negative feedback (NFB) loop is widely used for compensating both the temperature dependent fluctuation and the nonlinearity of the LED at the same time, as explained in the paper MJ2 of IOOC '81, San Francisco in U.S.A. (1981). In the wideband opto-electric NFB loop, nonlinear distortion of the LED is compensated by a negative feedback of a part of the LED light output to a drive circuit driving the LED after detecting the part of the light output of the LED. Then it is desirable to construct in a single body a light emitting element and a light detecting element in order to compensate for the nonlinear distortion.
A prior art composite optical semiconductor device, which includes in one body a LED and a photo detector (PD), is shown in FIG. 1. In the prior art device, the LED is constructed as follows. A P-type GaAlAs layer 3 is formed on an N-type GaAs current confinement layer 2 which is formed on a P-type GaAs substrate 1. The P-type layer 3 penetrates the current confinement layer 2 in the center area thereof. An N-type GaAlAs layer 5 is formed on an N-type GaAs active layer 4 which is formed on the P-type layer 3. The LED emits light in the active layer 4 by applying a voltage between a first electrode 6 and a second electrode 7. A PD comprises the N-type GaAlAs layer 5, a GaAlAs intrinsic layer 8 and a P-type GaAs layer 9. As the GaAlAs layer 8 detecting the light of the LED has a low impurity concentration, the PD has a P-I-N construction. The PD is operated by applying a voltage between a third electrode 10, formed on the P-type GaAs layer 9, and the second electrode 7. Namely, the N-type GaAlAs layer 5 and the electrode 7 are commonly used for both the LED and the PD. In this structure, light emitted from the active layer 4 is detected by the PD and is transmitted to the outside through a light transmitting portion 11 which is formed in the center of the device. The light output from the transmitting portion 11 enters into an optical fiber 12. The electrical output of the PD supplies an outside electrical circuit which is part of a wideband opto-electric NFB loop in order to control the LED driver circuit.
In the wideband opto-electric NFB loop, it is necessary to obtain an exact monitoring current dependent on the light emitted from the LED which passes to the PD. The PD detects this part of the light output of the LED. But it is difficult to get a correct monitoring current in the prior art device because in the prior art device, the electrode 7 is commonly used for both the LED and the PD and is constructed outside of the detecting portion of the PD. Thus, the current path of the LED is close to that of the PD, and both paths partly overlap each other. Therefore, a leakage current flows from the LED to the PD, and the PD may detect an erroneous monitoring current of the light of the LED.